1. Field of the Invention
The present invention relates to a broad area semiconductor laser device, and more specifically to a broad area, high power semiconductor laser device that is suitable for, for example, an optical source in a device for initializing an optical disk, a pump laser in an optical amplifier, or an optical source in a laser machining tool.
2. Description of Related Art
High power semiconductor laser devices, which radiate a high power laser beam, with an output level of, for example, 300 mW or greater, are gaining attention as semiconductor light emitting devices for optical sources in, for example, a laser machining tool, a device for initializing an optical disk, a laser printer, or as a pump laser in an optical amplifier.
A characteristic required of a high power semiconductor laser device that is used as an optical source in the applications mentioned above is uniformity in a near field pattern (NFP) optical intensity. If the NFP optical intensity distribution were not uniform, and an NFP profile were to include bright and dark areas with significant contrast differences, large ripples, or significant differences in the maximum and minimum optical intensity levels, then the semiconductor laser device would cause a printing non-uniformity, for example, in a laser printer, or a non-uniform initialization on the optical disk. Ideally, the semiconductor laser device should offer a uniform optical intensity distribution in the direction of the width of a laser stripe part in the NFP profile.
The non-uniformity in the NFP becomes noticeable when the broad area semiconductor laser device has a laser stripe part width that is greater than or equal to 20 μm. Japanese Patent Application Publication 2001-230493 describes a semiconductor laser device that has a uniform optical intensity distribution of the NFP profile in the direction of the width of the laser stripe part. In order to make the optical intensity of the NFP profile uniform in the broad area semiconductor laser device, a film thickness of a clad layer outside of a current injection region should be less than or equal to 0.7 μm.
A structure of a broad area AlGaAs semiconductor laser device of a prior art, which incorporates the invention described in Japanese Patent Application Publication 2001-230493, will be described by referring to FIG. 5. FIG. 5 shows a cross-sectional view of a structure of the broad area AlGaAs semiconductor laser device.
As shown in FIG. 5, a broad area GaAs semiconductor laser device 10 includes a laminated structure consisting of a n-Al0.5Ga0.5As first clad layer 14 an active layer 16 consisting of an AlxGa1-xAs optical guide layer and an AlyGa1-yAs quantum well layer, where x>y, a p-Al0.5Ga0.5As lower second clad layer 18 that has a film thickness of less than or equal to 0.7 μm or, for example, a film thickness of 0.3 μm, a GaInP etch stop layer 20, a p-Al0.5Ga0.5As upper second clad layer 22, and a p-GaAs contact layer 24, all of which are formed on top of a n-GaAs substrate 12.
The p-AL0.5GA0.5As upper second clad layer 22 and the p-GaAs contact layer 24 are etched down to the GaInP etch stop layer 20 and are formed into a ridge 26, which is in a shape of a ridge. The GaInP etch stop layer 20 is exposed on the sides of the ridge 26.
An n-GaAs layer 28 is formed in such a way as to bury the ridge 26 and the GaInP etch stop layer 20, which is exposed on the sides of the ridge. The n-GaAs layer 28 forms a current non-injection region with a pn junction isolation. Although not shown in this figure, a p side electrode is formed on top of the p-GaAs contact layer 24, and an n side electrode is formed on the back side of the n-GaAs substrate 12.
In, for example, a device for initializing an optical disk, a semiconductor laser device that functions as an optical source radiates a laser beam onto an optical disk in order to initialize the optical disk. A laser beam reflecting back from the optical disk can destabilize the output from the semiconductor laser device or degrade the semiconductor laser device. Therefore, the back reflection must be shielded.
To address this issue, a polarized beam splitter (PBS) is placed between the semiconductor laser device and the optical disk for redirecting the back reflection from the optical disk by a 90° angle and for splitting the laser beam that is emitted from the semiconductor laser device into the PBS into a P wave (a p polarized component) and a S wave (an s polarized component). While the P wave is allowed to travel straight, the S wave is deflected by 90° with respect to the incoming angle, so that the P wave, which has a high optical intensity, will radiate on the optical disk. While the P wave is an optical component that is polarized in the direction of the compound semiconductor layers that make up the semiconductor laser device, the S wave is an optical component that is polarized in a direction normal to the compound semiconductor layers in the semiconductor laser device.
Furthermore, the semiconductor laser device that is used as the optical source in the device for initializing the optical disk must offer flat profiles or top hat shaped profiles, for both the P wave and the S wave. Here, a flat or a top hat shaped profile refers to a profile with the same optical intensity levels at the various coordinate points in the direction of the width of the laser stripe part. Furthermore, assuming that the average value for the optical intensity levels at the various coordinate points in the direction of the width of the laser stripe part is A for the P wave (refer to FIG. 6A) and the maximum value for the optical intensity levels at the various coordinate points along the width of the laser stripe part for the S wave is B (refer to FIG. 6B), it is required that (B/A)×100 be 13% or smaller.